The present invention relates to a servo control apparatus of a disk recording system and particularly to a servo control apparatus and process capable of controlling the position of a head for recording and reading digital data on a disk as recording media.
In a current disk recording system, a hard disk drive (hereinafter referred to as HDD) operates in two modes, in accordance with the moving distance, the first of which is called a seek mode, in which a head moves between tracks so as to reach target position, and the second of which is called a track following mode, in which the head is accurately positioned on a data line of the track after the head has reached the target track.
In a method for controlling the position of the head in a hard disk drive, the control of velocity is executed until the head reaches a target position and the control of position is executed at the target position, so that the head lies on the track.
In a typical hard disk drive, the seek mode executes a search mode in the first deceleration phase and executes a transition mode in the second deceleration phase. Therefore, the seek mode is separated into the search mode and transition mode, in which servo information having the course of feedback uses a track number with a gray code. In the track following mode however, servo information uses a position error signal (hereinafter referred to as PES) using A and B bursts.
In such a conventional control method for respective seek (and its search and transition modes) and track following modes, a compensator or a proportional integral derivative controller has been frequently used. A voice coil motor controls the position of head in the disk recording system.
The conventional servo control apparatus may be an analog control apparatus depending upon the hardware to be controlled, or a mixed analog and digital mixing control apparatus. First, the head reads the number of the track in which the head is positioned currently through the gray code value indicating the servo information of track for each sampling period. As a result, the moving distance by which the head should be moved from a current position to a target position is defined. Target velocity corresponding to the moving distance is obtained from a look-up table. When the moving distance is defined, the seek mode is also determined. A real velocity is, however, determined by the difference between the value prior to sampling and current sampling value of the current position. Therefore, in the seek mode being separated as the search mode and transition mode, the control of velocity is executed by a control input derived from the target velocity and real velocity.
The conventional servo control apparatus as discussed above however, has the following problems which should be cleared. First, the real velocity is calculated for each sampling interval. The value calculated tends to be come more inaccurate however, where the sampling interval is short and the real position has changed substantially. Thus, there is a problem when the voice coil motor is required to execute an unexpected or abnormal movement in the search mode the head during the search mode or the transition mode.
Secondly, the track following mode which has a full state feedback which uses position information (a number of track, PES) and velocity information containing expectable errors, has great difficulties in maintaining the complete control due to instability of the real velocity information.
Third, it is possible that a steady state error will occur during the track following mode because there is no method for directly removing an external disturbance. While such an error is capable of being indirectly controlled by using an integral controller, but is not capable of being controlled over the whole range of data on the recording media, so that the error is not completely removed.
U.S. Pat. No. 5,182,684, to Thomas et al. for an Estimator Positioning System And Method, endeavors to control the movement of a head of a disk file by generating a position signal in response to servo information read. The current position signal is sampled regularly, and together with a current actuator control signal, updates estimated velocity. The distance from the current position to a specified position, ie., distance-to-go, is also updated regularly. When the distance-to-go is greater than seven tracks the control signal is generated according to a maximum velocity of the actuator. When a maximum velocity is reached the system switches to a coast mode or when the number of tracks is less than or equal to seven tracks the system switches to a deceleration phase. During the deceleration phase the actuator control signal is generated by squaring the estimated velocity and dividing the result by the distance-to-go. Thomas et al. also suggests using an estimated position instead of the actual current position signal. In practice, only the position information is calculated and is used to modify the distance-to-go at each sampling period. The deceleration phase is followed by a track-following mode when the distance-to-go is between 1.0 and 0.125 tracks, i.e, less than 0.25 tracks.
U.S. Pat. No. 5,164,931, to Yamaguchi et al. for a Method And Apparatus For Control Of Positioning, uses a system for controlling positioning of a recording/playback head. The head is initially controlled as a velocity control system wherein a target velocity is generated in response to a track number signal. The difference between the head velocity and the target velocity is determined and the difference is supplied to a power amplifier for controlling the actuator for the heads. When the head reaches the vicinity of a target position, the head is controlled as a position control system utilizing the values of the target position and the position error signal of the current position of the heads.
U.S. Pat. No. 5,126,897, to Ogawa et al. for a Magnetic Recording/Reproducing Apparatus Capable Of Reducing A Setting Time In A Head Positioning Control Mode,for a servo type magnetic head drive control device with the moving velocity of the head controlled according to the moving distance to a target position in response to servo data and target velocity data. The servo data is used to generated position information and velocity information. Using the position information and target position information for calculating a moving distance, target velocity information is generated, in accordance with the moving distance. The velocity of the head is controlled for moving the head to a predetermined position on the basis of the difference between the velocity information and the target velocity information. After a certain time period or when a predetermined position is reached, control of the head movement is changed to be dependent upon a central processing unit (CPU) for operating in a positioning control mode having reduced settling time. In the position control mode, the CPU detects the position of the head actuator/driver using the position error signal (PES) from a head position decoder. The velocity signal, generated in response to the position error signal, is also detected by the CPU. The CPU then uses the position error signal and the velocity signal to control the positioning of the head. Use of a velocity estimated by the CPU in the position control mode is also discussed, by using a control model predefined in the CPU.
U.S. Pat. No. 5,051,851, to Sakurai for a Method And Apparatus For Positioning Head On The Basis Of Premeasured Amount Of Displacement, describes an apparatus having a plurality of disk-dependent displacement data stored in a table. The apparatus detects the displacement of a head from a designated track of a magnetic disk and controls the positioning of the head in a radial direction in response to disk-dependent displacement data from the table according to the designated track and further in response to the detected head displacement.
U.S. Pat. No. 5,040,084, to Liu for Disk Drive system And Method, provides servo tracks on a magnetic disk and storing position data of each servo track in memory by measuring the servo signal on each track.
U.S. Pat. No. 4,980,876, to Abate et al., for a Single Stage Track Seek Method, uses a seek method which remains in a positioning mode throughout the seek process. In the seek process the sinusoidal position error signal of the head relative to a track center is utilized and compared to an ideal profile stored in a look-up table. The head is moved on the basis of the difference between the ideal profile and the actual position.
U.S. Pat. No. 4,954,907, to Takita for a Head Positioning Control Method And System, mentions controlling head movement based on a new measured head position, a new measured head velocity, the previously measured head velocity, the previously computed actuator control signal, and predetermined stored constants when generating a new actuator control signal.
U.S. Pat. No. 4,920,462, to Couse et al. for a Disc Drive Fine Servo Velocity Control And Method For Head Positioning Relative To A Disc, uses servo velocity control circuitry for positioning a head on a destination track, and servo position control circuitry for centering the head in the destination track. The system uses the track identifying Grey code detected on the disc for moving the head to a track near the destination track under control of the servo velocity control circuitry, and uses the position error signal calculated in response to the detected servo data on the disc for centering the head on the destination track.
U.S. Pat. No. 4,914,644, to Chen et al., a Disk File Digital Servo Control System With Coil Current Modeling, provides long seek mode, ie., moving a head from track to track, and a short seek mode, i.e., a track following mode. During initialization a state estimator in a microprocessor provides four estimated values, ie., estimated head position, estimated head velocity, estimated voice coil motor current and windage (a bias force compensation current). Following initialization a digitizing PES channel provides primary and quadrature position error signals to the microprocessor, which in turn computes the actual position of the head and an estimator error is calculated based on the difference between a predicted head position and the actual head position. Next, the microprocessor computes a velocity error signal and compares this velocity error signal to a predetermined threshold. The estimated head position and estimated head velocity are computed based on the estimator error, a predicted head position and a predicted head velocity. The control signal is function of velocity error signal, and the estimated head acceleration. Following the output of the control signal the predicted variables are updated. The control signal is converted to analog form and provided to an integrated power amplifier. The voice coil motor current output by the integrated power amplifier is feed back and summed with the analog control signal.
U.S. Pat. No. 4,835,633, to Edel et al. for a Disk File Digital Servo Control System With Compensation For Variation In Actuator Acceleration Factor, stores information about the head velocity, coil current, and PES sampling times, and uses the stored information to calculate, an acceleration factor for generating a digital control signal which is utilized to control movement of the head.
U.S. Pat. No. 4,697,127, to Stich et al., an Adaptive Control Technique For A Dynamic System, controls a voice coil motor in a seek mode and track follow mode, wherein the seek mode is performed until the position of the head is within a quarter track of the target track, at which time the track following mode is performed. In the seek mode a microprocessor uses position information and an estimated velocity to produce a control signal for generating the coil current used in controlling the voice coil motor. The position error signal and the known coil current are used to generate an estimated bias and the estimated velocity, wherein the estimated bias is used within the estimator to correct the velocity estimate. In the track follow mode the microprocessor combines position, integrated position and estimated velocity signals to produce a composite signal which represents the magnitude to be applied to the actuator voice coil.
U.S. Pat. No. 4,679,103, to Workman, for a Digital Servo Control System For A Data Recording Disk File, has a digital servo system which receives a digital head position error signal and a digital signal corresponding to the head actuator input signal, and outputs a digital control signal. The digital control signal is converted to an analog signal, integrated and amplified to produce the head actuator input signal. The control signal is calculated from estimated head position values, estimated velocity and an estimated actuator input signal required to compensate for bias forces. The estimated values are functions of respective predicted values.
The present invention has an advantage over the noted prior art discuses above by using a previous control signal, a plurality of prediction estimates, a table of target values and units of different measure in each of three modes of operation for achieving stabilized settling of the head on the target track.
The transition mode is executed prior to about sixteen tracks from the target position, and upon reaching the target position, the advance speed of the head becomes lower. Although the seeking time becomes longer, the transition mode is executed for the purpose of stabilize settling the head. However, the conventional transition mode recognizes the current position of the head using the gray code and executes the control of velocity with only track position information using the gray code, so that an accurate control can not be executed during the performance of the transition mode and a bad influence can be effected on settling the track following mode.